Track signal responsive to variable frequency

ABSTRACT

A system providing a signal of an approaching train wherein an inductive loop formed by the railway tracks on which the train rolls serves to change the frequency of an oscillator when a railway vehicle enters the inductive loop thereby changing its inductance. A circuit serves to detect the change in frequency and provides a signal of the approach of a railway vehicle.

0 United States Patent -1 191 1111 3,796,776

Rawlins 1 Feb. 5, 1974 1 TRACK SIGNAL RESPONSHVE T0 3,503,007 3/1970 Kutschback 331/65 x V ARIABLE FREQUENCY 3,398,275 8/1968 Hatke et a]. 246/249 X 3,576,525 4/1971 Prucha 340/38 L Inventor: Wendel-1 Rawlms, 913 Crow 3,504,236 3/1970 Miyagawa et al. 331/117 R Topeka, Kans. 66611 3,266,028 8/1966 Taylor et a1 340/38 L X [22] Ffled: July 1971 Primary Examiner-Gerald M. Forlenza [21] Appl. No; 162,201 Assistant Examiner-George H. Libman Related Application Data Attorney, Agent, or F1rmClelle W. Upchurch [63] Continuation-impart of Ser. No. 845,195, July 28,

1969, abandoned. [57] ABSTRACT A system providing a signal of an approaching train [52] U.S. C1 246/40, 331/117 R, 331/175 wherein an inductive loop formed by the railway [51] Int. Cl B611 1/02 tracks on which the train rolls serves to change the [5 8] Field of Search 246/40, 249; 331/117 R, 175, frequency of an oscillator when a railway vehicle en- 331/65; 340/38 L, 258 C; 317/147 ters the inductive loop thereby changing its inductance. A circuit serves to detect the change in fre- [56] References Cited quency and provides a signal of the approach of a rail- UNITED STATES PATENTS y vehlcle- 3,240,142 4/1966 Duckitt et al 317/ 147 X 14 Claims, 8 Drawing Figures A H H Fl F] I F1 F! R F1 F1 F1 I l /0- H ,7 //0 1 r K 1.1 U L! U U Ll U U Ll U A B /6 l4 SIGNAL INDUCTANCE 31 INTERPRETING seusmvs CIRCUIT OSCILLATOR PATENTEDFEB 51w 3.790176 SHEET 2 Bf 2 VOLTAGE FREQUENCY H0. 6 F/G". 7

I/N'VE/VTO/P WENOELL GZIMWL/NS BY UM W ATTORNEY TRACK SIGNAL RESPONSIVE TO VARIABLE FREQUENCY This application is a continuation-in-part of my application Ser. No. 845,195, filed July 28, i969, now abandoned.

The .present invention relates to apparatus for detection of an approaching train and more specifically pertains to apparatus for producing a signal when a railway vehicle enters a particular section of track.

Devices are known which serve to indicate the presence of a train on a given track section. One such device detects the presence of a train at any one of a number of locations and a transmitter is provided at each location which transmits a signal of a predetermined frequency to a receiver tuned to the frequency of its transmitter. Such a system is costly and requires complex equipment and each transmitter-receiver must be tuned to a different frequency. Another prior system utilizing an inductive loop by providing insulated joints in the track at two locations and by connecting the tracks together at each location through an inductor on each side of the insulator. When a train moves inside the loop, the change in the inductance of the loop is employed to provide an indication of the presence of the train. Such a system and similar prior art devices are sensitive to changes in resistance caused by wet tracks.

Accordingly, it is an object of the present invention to provide a simple and reliable train detection system which is unaffected by weather conditions and which may be installed at reasonable costs.

Another object of this invention is to provide an improved detection or signalling device which does not require any substantial modification of existing railway tracks.

Other objects and features of the invention will be appreciated and become apparent as the present disclosure proceeds and upon consideration of the following detailed description taken in conjunction with the accompanying drawing wherein an embodiment of the invention is disclosed.

In the drawing:

FIG. 1 is a diagrammatic view ofa loop detection system embodying the invention and employing a portion of railway tracks and serving to indicate the approach of a train from either direction;

FIG. 2 is a diagrammatic view of a similar system for indicating the approach of a train from one direction;

FIG. 3 is a schematic diagram of the inductance sensitive oscillator represented diagrammatically in FIGS. 1 and 2;

FIG. 4 is a schematic diagram of the signal interpreting circuit represented diagrammatically in FIGS. 1 and FIG. 5 is a graph of the output voltage of the signal interpreting circuit as a function of the input frequency;

FIGS. 6 and 7 are diagrams of waveforms produced by the oscillating circuit; and

FIG. 8 represents the voltage waveforms for two values of rail to rail leakage resistance.

Referring to FIG. 1, there is shown at 10 an electrical connection which joins the rails 11 and 12. This connection may be either a direct short or a low impedance capacitive type short. The detection means comprises an inductance sensitive oscillator 14 and a signal interpreting circuit 16 connected to the rails at 17 and 18.

The section of track between points 17 and 18 and the short 10 defines an inductive loop. The tracks are shorted at two locatons A and B so that the system will detect a train coming from either the area A or from the area B.

While a train is beyond or outside of the loop, the oscillator functions at its normal frequency. This frequency is transmitted to circuit 16 which may be connected to a relay or other means so that no signal is produced while oscillator 14 is functioning at its normal frequency. When a train or a railway vehicle moves inside the loop, the wheels and the axle provide an electrical connection across the track 11 and 12 and this electrical path serves to reduce the length of the loop thus changing the inductance of the loop. As the inductance of the loop changes, the frequency transmitted to interpreting circuit 16 by the oscillator 14 is altered. This change in frequency is employed to control a relay, or other devices, to provide an indication that the train is within the loop. This indication can be used to control the flashing of railroad crossing lights or to compute the distance, speed, or direction of the train.

A similar system is shown in FIG. 2 which produces a signal indicating the presence of a railway vehicle in one area viewed from points where conductors are connected to the tracks. The circuit in FIG. 2 is substantially like that of FIG. 1 except that the short at the area B is omitted and insulated joints 21 are provided in the rails 11 and 12 adjacent the connections 17 and 18 and remotely of the short 10 at the area A. Thus, the detection circuits 14 and 16 will produce a signal when a train moves inside the loop defined by short 10 and connections 17 and 18. This signal will cease when the train has moved beyond insulated joints 21. The shorts 10 are not necessary for proper functioning of the system but serve to limit the length of the track under surveilance and provide a reference for fail-safe operation.

FIG. 3 illustrates the inductance sensitive oscillator 14 and it comprises two pairs of complementary transistors. The first pair of transistors are shown at O1 and Q2. The base of O1 is connected through a resistor R13 and a diode 25 to ajunction 26. This junction is connected through a resistor R1 and a capacitor C2 to a junction 27. The junction 26 is also connected through a diode 29 and resistor R14 to the base of Q2. The junction of diode 29 and resistor R14 is connected through resistor R3 to ground. The junction 27 is connected to the second complementary pair of transistors Q3 and 04 through capacitor C1 and a resistor R8 to the collector of Q3 and through the condenser Cl and a resistor R9 to the collector of Q4. The emitter of Q3 is connected to a voltage supply 28 and the emitter of O4 is connected to ground. The base of O3 is connected through a resistor R4 to the collector of Q1 and the base of O4 is connected through a resistor R7 to the collector of Q2. The emitters of Q1 and Q2 are connected together and both emitters are connected to the supply 28 through a resistor R5 and to ground through the parallel connection of a resistor R6 and a capacitor C4. The junction of the emitters of Q1 and O2 is connected through diode D1 to the base of Q1 and through the reverse'p-n junction of diode D2 to the base of Q2. The junction of diode 25 and resistor R13 is connected through R to the supply 28. The zener diodes Z1, Z2, and Z3 are connected as shown for lightning protection. The representation at L indicates the loop induc tance of the track. R represents the rail to rail leakage resistance of the track, the value of which may change with changing weather conditions. The junction 27 is connected to one of the track rails at 17 and the other rail is connected at 18 to ground.

In describing the operation of the oscillator, FIG. 6 shows the output voltage waveform taken across output terminals 37 and 38 and FIG. 7 shows the general voltage waveform present at point 27 of the circuit. Referring to FIG. 3, point 27 will have a voltage A (FIG. 7) immediately after transistors Q1 and Q3 have been turned on and transistors Q2 and Q4 have been turned off. The voltage A is connected to point 26 through capacitor C2 and resistor R1 to drive transistors Q1 and Q3 further into conduction.

However, the voltage at point 27 decays through the loop inductor L at a rate determined by the value of the inductance. When this voltage has decayed to a value B (FIG. 7), the voltage at point 26 will have decayed to such a value as to begin turning off the transistors Q1 and Q3 and begin turning on the transistors Q2 and Q4. When Q2 and Q4 begin conducting, the voltage at point 27 will assume the value C (FIG. 7). This negative voltage is connected through capacitor C2 and resistor R1 to point 26 and, since it is negative, drives transistors Q2 and Q4 further into conduction and transistors Q1 and Q3 out of conduction.

This negative voltage will also decay through loop inductor L at a rate determined by the value of its inductance. When this voltage decays to a value D, the voltage at the point 26 will be of such a value as to turn off transistors Q2 and Q4 and turn on transistors Q1 and Q3. Point 27 will again be impressed with a voltage having the value A (FIG. 7), and the cycle will repeat.

The frequency at which circuit 14 oscillates is determined by the value of the loop inductance since this value determines the rate at which the voltage at point 27 decays. As the value of inductance decreases, the rate of decay increases resulting in an increase in oscillator frequency. Since the value of inductance is decreased by a train entering the loop, the change in oscillator frequency can be used to indicate the presence of the train in the loop. A circuit capable of responding to this change in frequency is the signal interpreting circuit 16 shown in FIG. 4.

One important feature of the invention is a means incorporated into the oscillator of FIG. 3 for compensating for changes in rail to rail leakage resistance. The effect of such a compensation means is to allow the circuit to react to changes in the inductance of L while preventing any undo reaction to changes in the value of R the inherent rail to rail leakage resistance, the value of which is dependent upon existing weather conditions.

The value of R changes in accordance with weather changes such that the value of R is high in dry weather and low in wet weather. Resistors R8 or R9 form a voltage divider with R depending upon whether Q3 or O4 is conducting. When the value of R is high, the voltage at point 27 is large, as seen in FIG. 8. Since the voltage at point 27 is large, the capacitor C2 charges rapidly until the point 26 reaches the transistion voltage where the non-conducting transistors (OI-Q3 or Q2-Q4) become conducting. However, if the value of R is low, the voltage at point 27 is smaller and capacitor C2 charges slowly because it begins charging from a relatively smaller voltage potential. Since C2 charges slowly, point 26 reaches the transistion voltage in substantially the same amount of time as where R has a larger value.

The net effect of the compensation circuit is to ensure that the transition voltage is reached in an amount of time substantially independent of the value of R Thus, the circuit will be sensitve to changes in the loop inductance but not to changes in R Capacitor C1, resistors R1, R2, R3, R13, and R14, and diodes 25, 29, D1 and D2 also enhance this compensation.

Turning now to FIG. 4, the output of the oscillator is connected to the input terminals 41 and 42 of the signal interpreting circuit 16. The terminal 41 is connected through condenser C3 and a resistor R10 to the base of a transistor OS. A diode 43 is connected between the junction of the resistor R10 and the base of Q5 and ground. The collector of O5 is connected through inductor 44 to a voltage supply 46 and the emitter of O5 is connected to ground. A resistor R11 is connected across the collector and emitter terminals of Q5. The output is taken at a junction 47, rectified by diode 48 and filtered by a resistor R12 and a condenser C6.

In operation, as the input is transmitted to Q5, it is alternately rendered conductive and non-conductive to store and discharge energy in the inductor 44. Thus, when the input is at its maximum voltage the transistor Q5 is conductive connecting the inductor 44 directly to ground. Current will then build up through the inductor and collector-emitter circuit of Q5 storing energy in the inductor. When the input goes to its minimum level, O5 is rendered non-conductive and the energy stored in inductor 44 is discharged through the diode 48 and the filter R12 and the condenser C6 to produce a direct current output.

If the input is of a low frequency, the transistor 05 is conductive for longer periods of time and thus more energy is stored in the inductor 44 so that when the transistor O5 is rendered non-conductive, more energy is discharged by the inductor 44 to produce a high direct current output. But when the input frequency is high, the transistor is conductive for shorter periods and thus there is less time for the inductor to store energy than there would be if the input frequency were lower so that with a higher frequency the inductor stores less energy. Thus with a high frequency input and when Q5 is rendered non-conductive, the inductor 44 discharges to produce a lower direct current output.

FIG. 5 depicts the output of this frequency-to-direct current converter as a function of frequency. It will be noted that as the frequency increases, the direct current output decreases. Thus, when a railway vehicle enters the loop, the oscillator frequency increases and the output of the frequency-to-direct current converter decreases. Ifa relay such as shown at 51 is used to control the railroad crossing signal lights, while there is no train in the area, the output of the frequency-to-direct current converter is high, energizing the relay to open the circuit 52 and de-energize the signal light 53. When a train enters the loop, the direct current output of the frequency-to-direct current converter decreases and the relay 51 is thereby de-activated so that its contacts will close the circuit 52 to the signal light 53.

While the invention has been described with reference to particular components and specific circuit arrangements for detecting the approach of a train, it will be appreciated that other elements and circuit arrangements may be employed. Such modifications and others may be made without departing from the spirit and scope of the invention except as set forth in the appended claims.

What is claimed and desired to be secured by Letters Patent is:

1. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track comprising, an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle enters the loop, oscillator means comprising first and second switching means, a power source connected to said first and second switching means, and a resistor-capacitor means connected to said first and second switching means and to said power source, said resistor-capacitor means being charged alternately through said first and then through said second switching means at a frequency determined by said track and said resistor-capacitor means, means connecting said inductive loop to said oscillator means so that said changes in the inductance of said loop changes the frequency of oscillation of said oscillator means, and frequency to direct current converter means responsive to the change in frequency of said oscillator means for producing a signal indicative of the presence of a railway vehicle on the section of track forming said inductive loop, said frequency to direct current converter means comprising a third switching means connected to said oscillator means and controlled thereby, an energy storage means connected between said third switching means and a power supply, said storage means storing energy when said third switching means is closed and releasing said stored energy when said third switching means is opened, and energy receiving means for said released energy.

2. A railway track circuit of claim 1 wherein said inductive loop comprises shorting means connecting the track rails at each end of said section of the track.

3. A railway track circuit of claim 1 wherein said energy storage means comprises an inductor.

4. A railway track circuit of claim 1 wherein said energy receiving means comprises filter means for smoothing said released energy into a direct current output signal, and said third switching means comprises a transistor.

5. A railway track circuit of claim 1 wherein the first switching means comprises a first NPN transistor and a first PNP transistor and said second switching means comprises a second PNP transistor and a second NPN transistor, each transistor having a base and an emitter and a collector, said oscillator means further comprising first means connecting the emitter of said first NPN transistor to the emitter of said second PNP transistor, second means connecting the collector of said first PNP transistor to the collector of said second NPN transistor, third means connecting the collector of said first N PN transistor to the base of said first PNP transistor, and fourth means connecting the collector of said second PNP transistor to the base of said second NPN transistor, and means connecting the resistor-capacitor means between the bases of said first NPN transistor and said second PNP transistor and said second connecting means to thereby produce oscillation of the oscillator means.

6. A railway track circuit of claim 1 wherein said inductive loop comprises, shorting means at one end of said section of track connecting the track rails, and an insulated joint in each rail at the other end of said loop.

7. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track wherein the track has a rail-to-rail resistance value subject to changing environmental conditions 5 comprisng:

an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle travels in the loop;

oscillator means connected to said loop and having an output the frequency of which depends upon the inductance of the loop, said oscillator means including compensating means having a voltage divider to compensate for changes in rail to rail leakage resistance due to environmental changes wherein the output frequency of the oscillator remains substantially constant upon changes in rail to rail leakage resistance; and

signal interpreting means responsive to the change in oscillator frequency resulting from a train entering into said loop for providing an output indicative of the presence of a train within the loop wherein said signal interpreting means comprises a frequency to direct current converter means including a first switching means connected to said oscillator means, an energy storage means connected between said first switching means and a power supply, said storage means storing energy when said first switching means is closed and releasing said stored energy when said first switching means is opened, and energy receiving means for accumulating said released energy.

8. The railway track circuit of claim 7 wherein said oscillator means comprises:

second and third switching means;

a power source connected to said second and third switching means; and,

connecting means connected between the power source and the second and third switching means for alternately energizing the second and third switching means.

9. A railway track circuit to detect the position of a railway vehicle entering into a predetermined section of track comprising an inductive loop formed by said section of track wherein the inductance of the loop changes with the position of the railway vehicle in the loop, oscillator switching means, a power source con nected to said switching means, and a resistor and capacitor means connected through said switching means to said power source, said capacitor means being charged alternately from said power source and ground through said resistor means and said section of track at a rate determined by said track and said resistorcapacitor means, level detector means that will change states as the voltage across said section of track approaches zero, and output means that show the change of state of the level detector means for indicating the position of a railway vehicle on said section of track according to the time between changes of state of the level detector means.

10. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track wherein the trak has a rail-to-rail resistance value subject to changing environmental conditions comprising:

an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle travels in the loop;

oscillator means connected to said loop and having an output the frequency of which depends upon the inductance of the loop, said oscillator means including compensating means to compensate for changes in rail-to-rail leakage resistance due to environmental changes wherein the output frequency of the oscillator remains substantially constant upon changes in rail-to-rail leakage resistance, said oscillator means including first and second switching means, a power source connected to said first and second switching means, and connecting means connected between the power source and the first and second switching means for alternately energizing the first and second switching means, said connecting means including resistor-capacitor means including said compensation means connected to said first and second switching means and to said power source, said resistor-capacitor means being charged alternately through said first and then through said second switching means at a frequency determined by said track inductance and said resistor-capacitor means; and

signal interpreting means responsive to the change in oscillator frequency resulting from a train entering into said loop for providing an output indicative of the presence of a train within the loop.

ll. The railway track circuit of claim 10 wherein said first and second switching means each comprises input and output terminals and said connecting means comprises first resistive-capacitive means connecting the output of said first switching means to said inductive loop, second resistive-capactive means connecting the output of said second switching means to said inductive loop, third resistive-capacitive means connecting said loop to the input of said first switching means, and fourth resistive-capacitive means connecting said loop to said input means of said second switching means wherein said first, second, third and fourth resistivecapacitive means comprise said compensating means.

12. The railway track circuit of claim 11 wherein said signal interpreting means comprises a frequency to direct current converter means including a third switching means connected to said oscillator means, an energy storage means connected between said third switching means and a power supply, said storage means storing energy when said third switching means is opened, and energy receiving means for accumulating said released energy.

13. In combination:

a. an inductive loop having a variable shunt resistance there across, said loop being subject to a change in inductance and including a predetermined section of railway track, the inductance of said loop being subject to change due to the variable length of said loop as a train traverses said section of track;

b. a power source;

0. switching means connected to said power source;

d. a first resistor and capacitor means connected to said power source through said switching means, said capacitor means being alternately charged from said power source and ground through said first resistor means and said loop, said capacitor means being charged at a rate dependent upon the inductance in said loop, said first resistor means and said shunt resistance comprising a voltage divider which compensates for the variable shunt resistance;

e. means, responsive to a predetermined level of voltage across said loop, for generating an output signal; and

f. a second resistor and capacitor means connected between said loop and said generating means, said second resistor and capacitor means being connected to ground to provide a current path to charge said second capacitor means.

14. The combination of claim 13 wherein said predetermined section of track is bounded on both ends by a shorting means across the rails of the track, said section of track having a pair of terminals connected thereto, one terminal being connected to one rail ofthe track through ground, and the other terminal being connected to the other rail through said switching means. 

1. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track comprising, an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle enters the loop, oscillator means comprising first and second switching means, a power source connected to said first and second switching means, and a resistor-capacitor means connected to said first and second switching means and to said power source, said resistor-capacitor means being charged alternately through said first and then through said second switching means at a frequency determined by said Track and said resistorcapacitor means, means connecting said inductive loop to said oscillator means so that said changes in the inductance of said loop changes the frequency of oscillation of said oscillator means, and frequency to direct current converter means responsive to the change in frequency of said oscillator means for producing a signal indicative of the presence of a railway vehicle on the section of track forming said inductive loop, said frequency to direct current converter means comprising a third switching means connected to said oscillator means and controlled thereby, an energy storage means connected between said third switching means and a power supply, said storage means storing energy when said third switching means is closed and releasing said stored energy when said third switching means is opened, and energy receiving means for said released energy.
 2. A railway track circuit of claim 1 wherein said inductive loop comprises shorting means connecting the track rails at each end of said section of the track.
 3. A railway track circuit of claim 1 wherein said energy storage means comprises an inductor.
 4. A railway track circuit of claim 1 wherein said energy receiving means comprises filter means for smoothing said released energy into a direct current output signal, and said third switching means comprises a transistor.
 5. A railway track circuit of claim 1 wherein the first switching means comprises a first NPN transistor and a first PNP transistor and said second switching means comprises a second PNP transistor and a second NPN transistor, each transistor having a base and an emitter and a collector, said oscillator means further comprising first means connecting the emitter of said first NPN transistor to the emitter of said second PNP transistor, second means connecting the collector of said first PNP transistor to the collector of said second NPN transistor, third means connecting the collector of said first NPN transistor to the base of said first PNP transistor, and fourth means connecting the collector of said second PNP transistor to the base of said second NPN transistor, and means connecting the resistor-capacitor means between the bases of said first NPN transistor and said second PNP transistor and said second connecting means to thereby produce oscillation of the oscillator means.
 6. A railway track circuit of claim 1 wherein said inductive loop comprises, shorting means at one end of said section of track connecting the track rails, and an insulated joint in each rail at the other end of said loop.
 7. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track wherein the track has a rail-to-rail resistance value subject to changing environmental conditions comprisng: an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle travels in the loop; oscillator means connected to said loop and having an output the frequency of which depends upon the inductance of the loop, said oscillator means including compensating means having a voltage divider to compensate for changes in rail to rail leakage resistance due to environmental changes wherein the output frequency of the oscillator remains substantially constant upon changes in rail to rail leakage resistance; and signal interpreting means responsive to the change in oscillator frequency resulting from a train entering into said loop for providing an output indicative of the presence of a train within the loop wherein said signal interpreting means comprises a frequency to direct current converter means including a first switching means connected to said oscillator means, an energy storage means connected between said first switching means and a power supply, said storage means storing energy when said first switching means is closed and releasing said stored energy when said first switching means is opened, and enerGy receiving means for accumulating said released energy.
 8. The railway track circuit of claim 7 wherein said oscillator means comprises: second and third switching means; a power source connected to said second and third switching means; and, connecting means connected between the power source and the second and third switching means for alternately energizing the second and third switching means.
 9. A railway track circuit to detect the position of a railway vehicle entering into a predetermined section of track comprising an inductive loop formed by said section of track wherein the inductance of the loop changes with the position of the railway vehicle in the loop, oscillator switching means, a power source connected to said switching means, and a resistor and capacitor means connected through said switching means to said power source, said capacitor means being charged alternately from said power source and ground through said resistor means and said section of track at a rate determined by said track and said resistor-capacitor means, level detector means that will change states as the voltage across said section of track approaches zero, and output means that show the change of state of the level detector means for indicating the position of a railway vehicle on said section of track according to the time between changes of state of the level detector means.
 10. A railway track circuit to detect the presence of a railway vehicle entering into a predetermined section of track wherein the trak has a rail-to-rail resistance value subject to changing environmental conditions comprising: an inductive loop formed by said section of track wherein the inductance of the loop changes when a railway vehicle travels in the loop; oscillator means connected to said loop and having an output the frequency of which depends upon the inductance of the loop, said oscillator means including compensating means to compensate for changes in rail-to-rail leakage resistance due to environmental changes wherein the output frequency of the oscillator remains substantially constant upon changes in rail-to-rail leakage resistance, said oscillator means including first and second switching means, a power source connected to said first and second switching means, and connecting means connected between the power source and the first and second switching means for alternately energizing the first and second switching means, said connecting means including resistor-capacitor means including said compensation means connected to said first and second switching means and to said power source, said resistor-capacitor means being charged alternately through said first and then through said second switching means at a frequency determined by said track inductance and said resistor-capacitor means; and signal interpreting means responsive to the change in oscillator frequency resulting from a train entering into said loop for providing an output indicative of the presence of a train within the loop.
 11. The railway track circuit of claim 10 wherein said first and second switching means each comprises input and output terminals and said connecting means comprises first resistive-capacitive means connecting the output of said first switching means to said inductive loop, second resistive-capactive means connecting the output of said second switching means to said inductive loop, third resistive-capacitive means connecting said loop to the input of said first switching means, and fourth resistive-capacitive means connecting said loop to said input means of said second switching means wherein said first, second, third and fourth resistive-capacitive means comprise said compensating means.
 12. The railway track circuit of claim 11 wherein said signal interpreting means comprises a frequency to direct current converter means including a third switching means connected to said oscillator means, an energy storage means connected between said third switching means and a power supply, sAid storage means storing energy when said third switching means is opened, and energy receiving means for accumulating said released energy.
 13. In combination: a. an inductive loop having a variable shunt resistance there across, said loop being subject to a change in inductance and including a predetermined section of railway track, the inductance of said loop being subject to change due to the variable length of said loop as a train traverses said section of track; b. a power source; c. switching means connected to said power source; d. a first resistor and capacitor means connected to said power source through said switching means, said capacitor means being alternately charged from said power source and ground through said first resistor means and said loop, said capacitor means being charged at a rate dependent upon the inductance in said loop, said first resistor means and said shunt resistance comprising a voltage divider which compensates for the variable shunt resistance; e. means, responsive to a predetermined level of voltage across said loop, for generating an output signal; and f. a second resistor and capacitor means connected between said loop and said generating means, said second resistor and capacitor means being connected to ground to provide a current path to charge said second capacitor means.
 14. The combination of claim 13 wherein said predetermined section of track is bounded on both ends by a shorting means across the rails of the track, said section of track having a pair of terminals connected thereto, one terminal being connected to one rail of the track through ground, and the other terminal being connected to the other rail through said switching means. 